`DESCRIPTION` file: Additional repositories

[fweber144]

Due to the deprecation of https://mc-stan.org/r-packages/, I guess we need to replace https://mc-stan.org/r-packages/ in the DESCRIPTION file (in lines

projpred/DESCRIPTION

Lines 85 to 86 in 6d945b0

	Additional_repositories:
	https://mc-stan.org/r-packages/

) by https://stan-dev.r-universe.dev/ (see https://github.com/paul-buerkner/brms/blob/7838419adee48971e35bdc3f27148ad21739ec3e/DESCRIPTION#L99C1-L100C37, for example).